1. Field of the Invention
The present invention relates to a method for preparing an aqueous dispersion of a developer and, in particular, to a method for preparing an aqueous dispersion of a developer which can provide recording paper substantially improved in the color developing density and color developing velocity of recorded images, and printability of the developing surface thereof; as well as a pressure-sensitive recording paper obtained by the use of a coating composition containing the aqueous dispersion of a developer.
2. Description of the Prior Art
Active clay has been called inorganic developer, while phenol resins of novolak type and metal salts of nuclear-substituted salicylic acid have been called organic developers and have widely been employed for making pressure-sensitive recording paper (see, for instance, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J.P. KOKOKU") Nos. Sho 42-20144 and Sho 51-25174). Any organic developer of this type is finely divided or finely dispersed in a medium which is commonly water, mixed with an inorganic filler, an adhesive or the like and then applied onto the surface of a substrate such as paper (see, for instance, J.P. KOKOKU No. Sho 48-16341 and Japanese Patent Unexamined Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 54-143322).
Incidentally, the metal salts of nuclear-substituted salicylic acids used as developers for pressure-sensitive recording paper (hereunder simply referred to as "developer(s)") are in general an amorphous solid having a specific softening point and is applied onto the surface of paper after dispersing in water. Therefore, it is quite desirable that developers be provided in the form of a water dispersion in which the developer has a desired particle size and which is thick and excellent in handling properties and safety.
However, when coarse particles of a developer is directly pulverized into fine grains in water containing a dispersing agent or the like with a ball mill or a sand grinder (sand mill), it is very difficult to obtain fine particles of a developer and the resulting dispersion becomes highly thixotropic and has low fluidability which in turn makes the handling thereof difficult. On the other hand, an emulsified dispersion having good fluidability even at a high concentration can be obtained by adding an organic solvent or a plasticizer to a developer to form a liquid product and then emulsified and dispersed in water containing a dispersing agent with a strong dispersing means. However, the dispersed particles comprise, in this case, liquid drops containing an organic solvent or a plasticizer, therefore, the particles grow into large particles and the particles agglomerate in the vicinity of the wall of a container and deposit onto the wall during storage over a long time period. Thus, an emulsion having sufficient stability cannot be obtained.
Some solutions for these problems have been proposed in particular in J.P. KOKAI No. Sho 63-173680 or Sho 64-34782 which discloses a method for preparing an aqueous dispersion of a developer containing emulsion particles having a desired particle size, having good fluidability even at a high concentration and good in storage stability. The method comprises dissolving a developer in an organic solvent, emulsifying and dispersing the resulting organic solution in an aqueous solution containing a dispersing agent and then heating the resulting dispersion to distill off and remove the organic solvent.
In this way, to heat a dispersion per se for removing the organic solvent is desirable from the viewpoint of desired uses of the developers and the stability of the resulting dispersion, but strictly speaking, these proposed methods suffer from some problems.
More specifically, stable dispersed state of the emulsified dispersion of a developer containing an organic solvent must be held at a high temperature for a long time period for completely distilling off and removing the organic solvent from the dispersion per se. For this reason, the dispersion must be a system which is an excellent protective colloid. However, such an excellent protective colloid system is in general highly foamable and correspondingly the space in a distillation vessel is occupied by stable foams during the distillation of the organic solvent which prevents rapid removal of the organic solvent and in a worst case, the operation for removing it would often be interrupted. On the other hand, if a dispersion system having low foaming properties is selected, the system is in general a poor protective colloid, a part of the dispersion is broken during the operation for removing the organic solvent, in turn excessively large aggregates of a developer are formed and thus the resulting dispersion is often practically unacceptable.
These two tendencies reciprocal to one another become more conspicuous, as the size of the distillation vessel increases, the desired particle size of the developer is small, an external force such as the strength of stirring is high and the concentration of the developer is high. This is a major obstacle in production of stable aqueous dispersions of this kind in an industrial scale.